webserver: add a watchdog.
webserver: add a watchdog.

The watchdog exits if it can't request / on the webserver. This
protects against deadlocks taking down the service.

The hard coded default is to check every 30 seconds for a response
within 5 seconds.

Change-Id: I77798ab82cc27c678b943479a3ce763c7fd411c1

google/zoekt,google/zoekt